RE: Sgurr Nan Gillean
I would say that it's on a slightly higher level to all of those but not really severe. Once you have made your way through Coire Riabhach and made your way through the boulder field you will reach the broader part of the South East ridge. Make a note of where you came up out of the coire as it could be quite disorientating when returning, especially if foggy.
As you ascend the ridge it will get narrower and steeper quite quickly with various sections of scrambling over walls of rock and up gullys. As you get higher the sense of exposure increases as it becomes very obvious that you're ascending to quite a pointy summit. With around 25m or so to go the ridge narrows massively and levels out slightly.
If you're comfortable with exposure you should have no problem at all but remember that the weather can (and will) change at the drop of a hat which you need to be prepared for. Unless you're prepared with the correct equipment returning is via the same route as the two other routes off the summit are much more serious ground. Navigating in the fog can also be difficult as your compass will not be very accurate due to the magnetic properties of the rock.
It's a fantastic walk with some of the most amazing views in the UK. I'd probably recommend another easier peak beforehand so that you can understand the kind of terrain the Black Cullin present (maybe Bruach na Frithe or Sgurr na Bannachdich).
It's worth the drive afterwards to a decent pub rather than the Slig though. The Old Inn at Carbost isn't too far away and is much better for a post walk pint.